1. Field of the Invention
The present invention relates to a nonlinear resistor employed in an arrester, a surge absorber, etc., i.e., a nonlinear resistor which includes zinc oxide as a main component and has a nonlinear resistance characteristic, and a method of manufacturing the nonlinear resistor.
2. Description of the Related Art
In general, an overvoltage protection instrument such as an arrester, a surge absorber, etc. has been employed in the power system, and a nonlinear resistor has been frequently employed in such overvoltage protection instrument. In particular the "nonlinear resistor" has a nonlinear resistance characteristic which exhibits an insulating characteristic at the normal voltage but exhibits a low resistance value when an overvoltage is applied to the nonlinear resistor. Thus, the overvoltage being superposed on the normal voltage can be removed by such nonlinear resistor. Therefore, the nonlinear resistor is extremely effective for the protection of the power system and the electrical machinery and apparatus. Such nonlinear resistor has a sintered body. The sintered body includes zinc oxide as a main component. The zinc oxide is mixed, granulated, formed, and sintered while adding at least one type metal oxide as an additive to achieve the nonlinear resistance characteristic. Insulating layers are also formed on side surfaces of each sintered body, and electrodes formed of aluminum, etc. are formed on an upper surface and a lower surface of the sintered body by an arc thermal spraying, etc.
Discharge energy withstand is set to the above-mentioned nonlinear resistor. Then, the nonlinear resistor is brought into breakdown mechanically or electrically if the discharge energy being applied to the nonlinear resistor exceeds this discharge energy withstand. As one of breakdown types of the nonlinear resistor caused when such nonlinear resistor absorbs the discharge energy, there is breakdown which is due to an electrode layer of the nonlinear resistor. More particularly, the nonlinear resistor comes to breakdown in the following cases. That is, there are cases where,
(1) if the nonlinear resistors are stacked, discharge is generated in voids between the stacked electrode layers since a surface of the electrode layer is not flat, whereby the nonlinear resistor comes to breakdown,
(2) if the voids are formed in the electrode layers, discharge is generated in the voids, whereby the nonlinear resistor comes to breakdown, and
(3) partial current concentration is caused in the nonlinear resistor to bring the nonlinear resistor into breakdown, because of shape of end portions of the electrode and voids formed in the electrode layers.
Under above situations, various techniques for improving a discharge energy withstand characteristic of the nonlinear resistor have been developed and proposed. For example, the technique for employing aluminum containing any of Mg, Ca, and Ti as electrode material has been disclosed in Patent Application Publication (KOKOKU) Hei 7-44087 filed by the applicant of the present invention. In addition, the technique for suppressing difference between a maximum value and a minimum value of a distance between an electrode end portion and a sintered body external peripheral edge, i.e., an eccentricity of a circular disk type electrode against the sintered body containing an insulating layer, to less than 1 mm has been disclosed in Patent Application Publication (KOKAI) Hei 3-125401.
In recent years, power demand has been increased greatly and a transmission system voltage has increased steadily correspondingly. If the transmission system voltage is increased, the discharge energy being applied to the nonlinear resistor cannot help increasing. Hence, it is requested for the nonlinear resistor to have very high discharge energy withstand.